John writes about the things he had heard and seen, so that his “joy may be complete”. He experienced joy as a result of sharing the hope he had, with those around him.
The message John shares includes the need to walk in the light. “God is light”, so in order to have fellowship with Him, we must be filled with that same light, enlightened of our need to confess our sins, and seek forgiveness.
John then explains that we have an advocate through him we can be forgiven. It’s important for us to follow Jesus’ words in order for us to know him, and in order to demonstrate the love of God. These words need to abide in us, to affect the core of our being, and to be seen in the way we treat each other with real love. So let us “walk in the same way in which [Jesus] walked”, reflecting the light of God’s character, experiencing the joy that comes from sharing our hope.